1. Field of the Invention
The present invention is directed to a read-out system for a luminescent storage screen of an x-ray diagnostics installation wherein the screen is stimulated by read-out radiation, with the resulting emitted light from the screen being detected line-by-line.
2. Description of the Prior Art
Luminescent storage screens containing a stimulable phosphor are known for use in x-ray installations for storing a latent x-ray image. The stored image is read-out of such screens by causing the screen to luminesce by scanning with a stimulation radiation source, such as a bank of light emitting diodes arranged side-by-side. A detector acquires the light emitted by the storage screen, with the brightness values obtained thereby being used to construct and display a visual reproduction of the stored image.
It is also known to read-out such luminesce storage screens using a laser beam. The laser beam can be deflected in an appropriate scanning pattern with oscillating mirrors or a rotating polygonal mirror. During read-out, each picture element (pixel) is, in succession, exposed to an intense, focused laser beam, and the light emitted by the pixel due to this stimulation is acquired in a detector system. The detector system does not have topical resolution. A plurality of n picture elements are resolved per measuring line in a time T, so that only a small energy amount, of a maximum E=P.T/n is used per picture element, given a known laser power P. The light quanta triggered during photostimulation are dependent on the energy amount E. Given barium-fluorine-halogenide phosphors which are generally standard, this value lies between 10 through 200 nWs. Given a prescribed scan time of a few microseconds per picture element, light powers of approximately 10 mW are necessary. Because of the necessary deflection of the scanning beam, such light powers can generally only be achieved by lasers.
A read-out system using light emitting diodes as the stimulating radiation source is disclosed in European Application 0 144 963, wherein four light emitting diodes are arranged side-by-side, which are individually energized in succession. The respective scan beams from the light emitting diodes are deflected by a mirror such that each light emitting diode sweeps a defined region of a scan line on the luminescent storage screen. One picture element of a scan line is excited at a time, and emits light due to the successive stimulation by the light emitting diodes. The generated light is conducted by a light conductor onto a detector, which generates an electrical signal corresponding to the brightness of the emitted light of the picture elements. The signal is then used is a known manner to construct a visual display of the stored x-ray image. The relatively complicated mechanics for deflecting the scanning beams in read-out systems of this type is a disadvantage. Moreover, each picture element is excited only once by the light emitting diodes, so that only a slight light yield is obtained per picture element.
Another read-out system is described in European Application 0 191 322, corresponding to U.S. Pat. No. 4,703,177. This patent discloses a system for scanning a stimulable phosphor foil with a line of individual radiation sources. The radiation sources are individually driven such that picture elements which do not lie next to each other are caused to luminesce. With a corresponding light conductor, the light emitted by the individual picture elements is forwarded interleaved to different detectors, so that successively excited picture elements are respectively allocated to a different detector. Because the light emitting diodes in this system are individually driven, the problem of the power of the emitted light is also present, for a prescribed scan time.